Monitoring devices with embedded infrared sensors and detectors are frequently used for video surveillance of people and premises, fire detection, emergency responses, and various other applications where there is a need for such sensors and detectors. Detectors generally operate by detecting the differences in the thermal radiance of various objects in a particular scene. These differences in thermal radiation are converted into electrical signals which are processed, analyzed, and displayed as images in the case of video imager's and noise signals for detectors.
The use of bolometers as infrared detectors and imagers are well known in the art. In many cases the sensor array is a microbolometer array. When a microbolometer array absorbs infrared radiation of objects and their surroundings, there is a corresponding change in electrical resistance generated by a change in the microbolometer temperature. When used as an infrared detector or imager, the change in electrical resistance of the bolometer material resulting from the temperature change due to absorption of infrared radiation, is measured and recorded. Bolometers and microbolometers therefore generally act as resistive thermometers.
Cooled microbolometer detectors are costly to fabricate, heavier in weight, have shorter lifetimes, low yields, and consume considerably more power than uncooled microbolometers. Besides being lighter in weight, the uncooled microbolometers are cheaper to fabricate and consume less power than the cooled microbolometers. Uncooled microbolometers are currently used in the manufacture of highly specialized thermal imaging applications such as night vision and Scanning Thermal Microscopy (SThM) [1, 2, 3]. Two types of SThM microbolometers have shown considerable promise: Doped silicon microbolometers with temperature coefficient of resistance (TCR) between 0.003/K and 0.0056/K that are integrated into single-crystal silicon cantilevers [10, 11], and metallic microbolometers with a TCR around 0.0029/K [12].
The effective operation of microbolometers as infrared sensors and detectors requires them to have a high temperature coefficient of resistance (TCR) and low noise characteristics. The temperature coefficient of resistance (TCR) or α is the ratio of increased conductor resistance per degree Celcius rise in temperature of the conductor material. A material with a large value of TCR provides the best sensing capability. A higher sensitivity material is required to achieve the larger values of TCR. With pure metals, the TCR is a positive number because the resistance of these metals increases with increasing temperatures. Therefore, when bolometers and more particularly, microbolometers are used as infrared imagers to measure electromagnetic radiation emitted by surrounding objects, the efficiency of these imagers depend to a large extent on the metal used in the imagers. Lower resistivity bolometer film materials often have lower TCR values.
Microbolometers are generally fabricated on a substrate material using integrated circuit fabrication techniques. Adequate signal-to-noise ratio is essential for image processing and display. The signal to noise ratio as well as the response time and sensitivity of the bolometer depends on the thermal mass and thermal isolation from the supporting structure. The response time of a microbolometer is the time required for a detector to absorb sufficient infrared radiation to change the electrical resistance of the detector element accompanied by the dissipation of heat generated by the absorption of the infrared radiation. Microbolometer sensitivity on the other hand is determined by the amount of infrared radiation necessary to cause sufficient change in the electrical property of the microbolometer detector. With increase in thermal mass there is a decrease in sensitivity and increase in response time. Therefore, the thicker, the bolometer film material, the poorer, the overall performance of the imager.
The common semiconductor materials used in microbolometers are vanadium oxide (VOx) amorphous silicon (a-Si), and titanium oxide (TiOx). Some of the earlier bolometers used vanadium oxide as the semiconductor material (U.S. Pat. No. 5,450,053). Vanadium oxide has a TCR of approximately, 0.05/K and a superior noise equivalent temperature difference (NETD). However, vanadium oxide introduces a significant number of deposition problems [6,7,8]. U.S. Pat. No. 6,836,677 discloses a bolometer with a TCR higher than the conventional bolometers. The bolometer according to this disclosure uses a thin film of a crystalline or polycrystalline oxide selected from alkaline and rare earth elements, and one or more elements belonging to Period 5 or Period 6 of the Periodic Table. Polycrystalline and amorphous silicon have a high TCR of up to 0.05/K, but exhibit adverse noise characteristics [4,5,6]. In addition, microbolometers fabricated from vanadium oxides (VOx) and amorphous silicon (a-Si) have been shown to exhibit permanent changes in their electrical properties and in some cases mechanical deformation with a rise in temperature, even if temporary, resulting in changes in TCR values. The use of pure titanium and its alloys result in a bolometer that occupies a significant amount of space as described in U.S. Pat. No. 5,698,852. U.S. Pat. No. 7,442,933 discloses a bolometer that has substantially high resistance stability and substantially low 1/F noise. The bolometer in this patent comprises a substrate and a TiOx layer formed over the substrate, where the x value of the TiOx layer is in the range of 1.68 to 1.95.
Microbolometers have to be fabricated at temperatures compatible with Complementary Metal-Oxide-Semiconductor (CMOS) technology. In addition, the materials used to manufacture microbolometers must be inexpensive and compatible with current CMOS processes. Thin film metallic microbolometers have very low noise characteristics in addition to a low TCR of 0.005/K [6,9]. Thin film metallic microbolometers have other important advantages as well, including, simplified fabrication and a lower manufacturing cost. Metallic microbolometers also enable the use of alternative substrate materials such as polymers that tend to exhibit higher compliance properties and improved thermal isolation for better temperature resolution. U.S. Pat. No. 7,527,999 discloses the manufacture of a microbolometer film material Cd1-xZnxS, with a TCR in the value ranges from 1.5% to 3.7% for use at temperatures compatible with CMOS technology.
As a metal film is being deposited, the electrical properties of granular metals vary continuously as the composition of metal and non-metal mixtures is changed [13]. Metal deposition goes through the following four phases before it starts behaving like a bulk film. The first phase is nucleation which is the clustering of atoms and molecules. During nucleation, the film is highly susceptible to environmental and deposition conditions and substrate surface conditions. The second phase is island formation where stable nuclei grow and appropriate other nuclei. The third phase is when the islands combine to form networks of a few islands in contact with each other. This phase along with the first two phases are considered discontinuous. Finally, the film becomes continuous, but porous as some channels between the networks get filled [14-17]. Typically, island films have negative TCR while porous films have a positive TCR like bulk materials. The TCR for porous films is lower than that of thick bulk film [14, 15, 18, 19, 20]. Unlike bulk films, ultrathin films are comprised of a series of metal islands or grains, rather than a continuous film. The transport between films is due to tunneling or hopping. A small change in distance between the grains due to bending, results in a large change in resistance of the film. Accordingly, the gauge factor of these films is higher. In the present invention, cantilever sensors for sensing bending include metal sensors based on discontinuous films.
The primary conditions that have to be met as part of the continuing effort to develop, smaller, better performing microbolometers that weigh less and consume less power are, high temperature coefficient of resistance (TCR), low noise, inexpensive material, and compatibility with current Complementary Metal-Oxide-Semiconductor (CMOS) processes. The microbolometer of the present invention is fabricated taking into consideration these conditions and fills the deficiency gap for these conditions in the prior art.
An object of the present invention is to present the design of suspended beam sensors made of granular metallic, metal oxides such as, indium oxide, tin oxide, zinc oxide, or semimetallic thin films in the discontinuous phase. These sensors are used either as bolometers or as displacement sensors for sensing movement. Another object of the present invention is to present fabrication methods for these sensors from thin films that are close to the metal insulator transition (MIT) regime.